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Tribological Properties of Polytetrafluroethylene Composites Filled with Rare Earth Modified Carbon Fiber

Author: ShangGuanZuoZuo
Tutor: ChengXianHua
School: Shanghai Jiaotong University
Course: Mechanical Design and Theory
Keywords: polytetrafluoroethylene (PTFE) carbon fiber composites rare earths interface tribological properties
CLC: TB302.3
Type: PhD thesis
Year: 2007
Downloads: 710
Quote: 4
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Abstract


Polytetrafluoroethylene (PTFE) is an ideal self-lubricating material which has very low friction coefficient, high thermal stability and chemical stability. But it can not be used as anti-wear material alone because of its poor mechanical properties, bad thermal conductivity and high wear rate. Carbon fibers (CF) are widely used as fillers in composite materials due to their high specific strength, high modulus and high resistance to fatigue and creep. However, their applications have been largely limited because of their chemically inert surfaces, low wettability and interfacial bonding with resin matrix.In order to improve the interfacial adhesion and interfacial toughness between CF and PTFE matrix, rare earths (RE) were used to modify carbon fiber surface in this dissertation, and then, enhance the tribological properties of the CF reinforced PTFE (CF/PTFE) composites. Some significant original results were obtained.Firstly, rare earths modifier and air oxidation treatment were used for the surface modification of carbon fiber, and the effects of these two methods on CF/PTFE composites were comparatively evaluated through tensile strength and bending strength tests. The optimum rare earths content in the rare earths modifier was also discussed. The results showed that RE surface treatment was superior to air oxidation in promoting interfacial adhesion between carbon fiber and PTFE matrix. The interfacial adhesion of the RE surface treated CF/PTFE composites was mainly controlled by the contents of RE, and the optimum mechanical property was obtained when the content of rare earth elements was 0.3 wt%.Secondly, X-ray photoelectron spectroscopy (XPS) was used to analyze the mechanism of fiber surface treatment. The results showed that, with RE surface treatment, the CC bond of carbon fiber was loosed because of the affinities between RE and carbon. Therefore, the surface activity of carbon fiber was improved, and the amount of oxygen-containing functional groups on carbon fiber surfaces was also increased; Coordination chemical reactions occurred among RE, oxygen-containing groups and PTFE molecules, leading to an improvement in the interfacial bonding and toughness of CF/PTFE composite, which accordingly improved the mechanical properties of the composite.Thirdly, CF/PTFE composites were manufactured by incorporating differently surface treated carbon fiber into PTFE resin. The friction and wear behaviors of the composites under continue sliding friction were investigated in detail. The results showed that, under the same experimental conditions, the tribological properties of the PTFE composites filled with rare earth treated carbon fibers were better than those of PTFE composites filled with untreated and air-oxidated carbon fibers. The friction coefficient and wear of the CF/PTFE composites were obviously decreased under oil lubrication condition.Fourthly, under reciprocating sliding condition, the high interfacial bonding and toughness of RE treated CF/PTFE composite effectively resisted the impact caused by applied load. The large-scale transfer and rubbed-off of PTFE were restrained, which reduced the friction and wear of CF/PTFE composite.Fifthly, the improvement mechanism of RE on tribological properties of CF/PTFE composite was discussed. After RE surface treatment, the improved interfacial bonding and toughness advanced the reinforcement of CF and the load-carrying capability of CF/PTFE composite. The RE triochemical reaction promoted the formation of transfer films on the counterface. Under dry sliding condition, the productions of RE triochemical reaction acted as solid lubrication and reduced the friction and wear of the composite; under oil lubrication, these productions and RE complex on CF surfaces became the additives of lubrication oil, and improved the lubricating effect of the oil.Based on the development of high performance CF/PTFE composite, the process of RE surface treatment of carbon fiber and the mechanism of rare earths on interfacial obdurability of CF/PTFE composite were discussed. The effects of RE surface treatment on mechanical and tribological properties of CF/PTFE composite were systematically investigated. Developing the new surface modification technology of carbon fiber with rare earths is very important for improving engineering value of CF/PTFE composites and surface engineering application of rare earths.

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